A photo diode has been widely used as a photoelectric conversion device by using a p-n junction, in which a DC voltage is generated by photovoltaic effect due to electron and hole generated by a photon absorbed by a junction layer of the photo diode. As new renewable energy becomes more important socially due to global warming and depletion of fossil fuels, expectations of a photoelectric conversion device capable of directly converting clean and inexhaustible solar energy into electric energy has been increased. Accordingly, studies for improving energy conversion efficiency of a photoelectric conversion device have been actively conducted.
Further, in order to improve energy conversion efficiency of a photoelectric conversion device, studies for improving light absorption efficiency in a photo diode or separation and collection efficiencies of electron and hole generated by an absorbed photon in a photo diode have been actively conducted. As a representative approach for improving light absorption efficiency of a photo diode, there is an attempt to substitute silicon (Si) which is an indirect transition-type material widely used as a material of a photo diode, with a direction transition-type compound semiconductor material. Recently, in order to obtain a high light absorption efficiency, there is an attempt to implement a multi-junction solar cell (or tandem cell) which is improved in light absorption efficiency in the whole wavelength range of sunlight by using a wide band-gap material as a surface and heteromaterials gradually decreased in band gap toward the inside of a photo diode as a multijunction structure so as to absorb photons of energy corresponding to the respective junctions in a selective and parallel manner. However, in order to implement a photo diode with a multijunction of heteromaterials, a complicated manufacturing process is needed, and, an interface defect may easily occur at a junction interface due to a lattice mismatch, which results in that a dangling bond of an interface defect induces recombination of electron and hole so that there is a limitation in materials suitable for a lattice match that induces epitaxial growth for high efficiency.
As a method for improving light absorption efficiency in addition to the multijunction structure, there is a method of forming an intermediate band in a forbidden area of an energy band gap to use photons by sequentially absorbing the photons from a valence band to a conduction band through the intermediate band, which results in a high light absorption efficiency with respect to photons in the whole wavelength range of sunlight. This method is applied to an intermediate band solar cell.
In order to improve charge separation and collection efficiencies, there is an attempt to reduce an interface defect at an interface between a light absorption layer and a passivation layer formed between a base, an emitter and a diode, and to increase a mobility of electron and hole, and this attempt can be easily found in polycrystalline materials or heterojunction materials. Further, in a conventional photo diode structure, each of n-type, p-type, and intrinsic materials has a microscale thickness, and thus an energy band is bent only at a junction. Therefore, the conventional photo diode structure has a fundamental limitation in that only electron and hole generated by photons can be easily moved at a junction interface by a potential difference.
The recent trend of study is to apply two-dimensional materials to photoelectric conversion, in which the two-dimensional materials have two-dimensionally perfect single crystal structure to have no junction defect and also have a high photocharge separation and collection efficiency since an energy band is bent in the entire region due to the low-dimensional structure. Further, a junction in a two-dimensional material causes an energy band to be bent at a small thickness, which results in that a sharp bending of the energy band caused by the small thickness enables an electron to move to a conduction band by tunneling and thus may increase light absorption efficiency.
Meanwhile, graphene, which is the most well known as a two-dimensional material, does not have a band gap by Klein tunneling and thus cannot be applied as a material of a photo diode. Accordingly, regarding application of a two-dimensional structure to a photoelectric conversion device, studies for applying molybdenum disulfide (MoS2) or tungsten diselenide (WSe2) which shows an energy band gap when exfoliated into a single layer, as a p-n junction atomic layer photo diode are mainly conducted.
Korean Patent Laid-open Publication No. 10-1989-0011102 discloses a method for forming a shallow junction, including: forming a film including a hydrogen compound containing one device selected from the group consisting of boron, phosphorous, and arsenic to a thickness of several atom layers to 1,000 Å on a silicon substrate; and annealing the film, whereby an impurity region having a depth of 1000 Å or less and an impurity concentration of 1018 to 1021 is formed in a surface layer of a silicon layer.